All
attachments: http://www.atmos.albany.edu/student/heathera/mapdisc_03-23-07/
Hi
Folks,
The focus of Friday map discussion for
"beautiful strong of pearls" in a separate post to the
tropical-storms bulletin board. On of these "pearls," TC Gamede, proved to be a real "gem" because it
established new world-record rainfalls for 3-day through 9-day amounts on the
small, mountainous French island of La Reunion located near 21 S and 55 E (~800
km east of Madagascar). Although TC Gamede never
struck La Reunion directly (it passed
within
200 km to the NW of the island), outer rainbands
associated with the slow-moving storm were over the island for days. I wonder
if Louis XV had these future rainfall records in mind during his infamous
"rain" when he said "apres moi, le deluge"?
The following imagery is attached:
1.
IR satellite image and 850 hPa analysis for the
2.
Mean precipitable water, mean and anomalous 850/200 hPa winds for 16-25 Feb'07
over the southern Indian Ocean, and selected Hovmoller diagrams of the 1000/700
hPa meridional wind for Feb'07 (all imagery was derived from the NOAA/CDC
interactive web page).
3.
Selected 850/200 hPa streamfunction, nondivergent
wind and relative vorticity analyses centered on the southern Indian Ocean
during the period of interest (imagery was derived from the below web link
posted by Heather Archambault).
Other imagery (non shown) from the Ron
McTaggart-Cowan high-resolution GFS animation builder were also examined (link
given
below).
A cursory analysis of the available
imagery suggests that:
1.
The "string of TC pearls" formed along a corridor of 10-day time-mean
PW values between 52.5 and 60.0 mm. Caveat: these 10-day time mean PW values
are likely influenced by the presence of the TCs.
2.
The high PW corridor coincides with a band of anomalously strong 850 hPa easterlies
that runs from extreme northern Australia WSW to
3.
Anomalous time-mean 850 hPa cyclonic vorticity is implied along the equatorward
side of the anomalously strong aforementioned easterlies.
4. A
back-of-the-envelope estimate of the 10-day time-mean meridional vorticity
gradient suggests that the necessary condition for barotropic instability, a
reversal of the sign of the meridional vorticity gradient, occurs in several
longitude bands. If this estimate is supported by a more careful calculation
then the possibility exists that vortex formation along the band of cyclonic
vorticity may have received a boost from barotropic instability.
5.
The 10-day time-mean 200 hPa winds suggest that the subtropics over the
southern
6.
The 10-day time-mean anomalous 200 hPa winds show the existence of a prominent
anomalous anticyclonic circulation centered near 30 S and 60 E to the east of
the aforementioned higher-latitude trough. A similar, but weaker, anticyclonic
circulation is located near 30 S and 110 E. These anticyclonic circulation
features suggest the likely importance of tropical heating anomalies on the
evolving circulations. Strong (15 m/s) outflow across the equator into the NH
is indicated near the coast of eastern
7.
The 200 hPa stream function, nondivergent wind and
relative vorticity analyses shown for 14, 16 ,19 and
26 Feb'07 indicate that narrow vorticity strips extending westward and
equatorward from the equatorward tails of midlatitude troughs appear to
interact with some of the TCs during part of their life cycle and also
influence the recurvature and likely extratropical transition of one of the TCs
toward the end of the period of interest. The extent (if any) to which these PV
tails in conjunction with the possible barotropic instability play a role in in situ TC development needs to be investigated.
8.
Collectively, the 850/200 hPa stream function, nondivergent
winds and relative vorticity analyses suggest that any midlatitude influences
on TC development (especially Favio) most likely
occurred early on between 20-60 E from coastal southeastern Africa eastward to
la Reunion and Mauritius islands. Likewise, the signature of the interaction of
tropical disturbances with the mountains of
9.
Hovmoller diagrams of the 1000/700 hPa v-wind component between 10-20 S for
Feb'07 suggests the existence of two westward-propagating vorticity anomalies
that originate east of 110-115 E near 12 and 18 Feb'07, respectively. Both of
these westward-propagating disturbances tend to become quasi stationary near 60
and 80 E, respectively, by
10.
A Hovmoller diagram of 700 hPa heights between 30-40 S for Feb'07 shows the
presence of a westward-propagating disturbance that moves from 0 to 50 E
between 7-15 Feb'07. Downstream development beginning with a ridge eruption
near 70 E on 15 Feb and trough development between 100-120 E
on 16-17 Feb is also suggested. The possible connection, if any, between
a trough near 85-110 E on 10-11 Feb and the westward-propagating trough between
10-120 S mentioned in 9) needs to be investigated.
Lots
of bait has been cast. Further "pearls" of wisdom from outraged
recipients of this email post are welcome.
Lance
http://www.atmos.albany.edu/student/heathera/atm631.html
http://www.atmos.albany.edu/facstaff/rmctc/DTmaps/animSelect.php